bigdist: Store Distance Matrices on Disk

Provides utilities to compute, store and access distance matrices on disk as file-backed matrices provided by the 'bigstatsr' package. File-backed distance matrices are stored as a symmetric matrix to facilitate out-of-memory operations on file-backed matrix while the in-memory 'dist' object stores only the lower diagonal elements. 'disto' provides an unified interface to work with in-memory and disk-based distance matrices.